Abstract
In back flashover analysis, it is necessary to clarify how the grounding of transmission line tower and electric facilities responses to lightning surge. Although the grounding of transmission line tower has several ohms in low frequency range (dc-power frequency) as the ordinary grounding resistance, we adapt several tens ohms in lightning surge analysis generally. Even if the grounding impedance is reduced less than several ohms in low frequency range for high soil resistivity, it is not proved that the reduction of the grounding impedance is effective in high frequency range as lightning surge. We investigated the surge response of the tower grounding by numerical electromagnetic field analysis and time domain reflection method under the condition that the ground has infinite or finite conductivity. A cylindrical tower model showed almost zero grounding surge impedance for perfectly conductive ground. A four-flames tower model, however, of which flames extend from the top to the base showed the capacitive grounding surge impedance about 20 Q even if the ground is perfect conductor. This is caused by the leaning current paths formed by flames. The grounding mesh structure made by horizontal electrodes also had the surge impedance about 20 Ω and the capacitive surge response. These results show that a grounding system has a lower limit of the surge impedance when we try to reduce the impedance by wayy of the the prevention of back flash over for lightning surge.
